Using the radio-iodinated human serum albumin ([131-I]-RISA) dilution method to measure lamb fetal pulmonary fluid (FPF) volume, we followed the disappearance of protein-complexed, 14-C-labeled phosphatidylcholine ([14-C]PC) during the first 90 min after its injection into FPF. The FPF samples were analyzed for total lipid 14-C activity and for distribution of 14-C in PC, other phospholipids (PL), fatty acids (FA), and neutral lipids (NL). For most sampling periods ascending aortic (AAo) and right atrial (RA) blood samples were obtained simultaneously with FPF and serum was analyzed for total lipid 14-C activity and for distribution of 14-C in total PL, FA, and NL. These studies indicate that (1) PC is cleared rapidly from FPF with an estimated half-time of 15-57 min; (2) FPF-PC may be metabolized to lyso-PC and FA within the fluid itself; and (3) FA derived from PFP-PC enter the pulmonary circulation, thus establishing a pulmonary arteriovenous FA gradient. The possible sites at which PC may be cleared from FPF are considered.